The following meanings apply in what follows:
“Meridian plane” is a plane containing the axis of rotation of the tire.
“Equatorial plane” is the plane passing through the middle of the tread surface of the tire and perpendicular to the axis of rotation of the tire.
“Radial direction” is a direction perpendicular to the axis of rotation of the tire.
“Axial direction” is a direction parallel to the axis of rotation of the tire.
“Circumferential direction” is a direction perpendicular to a meridian plane.
“Radial distance” is a distance measured at right angles to the axis of rotation of the tire and from the axis of rotation of the tire.
“Axial distance” is a distance measured parallel to the axis of rotation of the tire and from the equatorial plane.
“Radially” means in a radial direction.
“Axially” means in an axial direction.
“Radially on the inside of or radially on the outside of” means at a shorter, or longer, radial distance.
“Axially on the inside of or axially on the outside of” means at a shorter, or longer, axial distance.
A tire comprises two beads that provide the mechanical connection between the tire and the rim on which it is mounted, the beads being respectively joined, by two sidewalls to a tread intended to come into contact with the ground via a tread surface.
A radial tire more specifically comprises a reinforcement comprising a crown reinforcement, radially on the inside of the tread, and a carcass reinforcement, radially on the inside of the crown reinforcement.
The carcass reinforcement of a radial tire for a heavy vehicle of the civil engineering type usually comprises at least one carcass reinforcement layer made up of metal reinforcing elements coated with a coating polymer material. The metal reinforcing elements are substantially parallel to one another and make an angle of between 85° and 95° with the circumferential direction. The carcass reinforcement layer comprises a main portion, that joins the two beads together and is wound, in each bead, around a bead wire core. The bead wire core comprises a circumferential reinforcing element usually made of metal, surrounded by at least one material which, nonexhaustively, may be made of polymer or textile. The winding of the carcass reinforcement layer around the bead wire core goes from the inside towards the outside of the tire to form a turned-back portion of carcass reinforcement comprising an end. The turned-back portion of carcass reinforcement, in each bead, anchors the carcass reinforcement layer to the bead wire core of that bead.
The end of the turned-back portion of carcass reinforcement is, on its two respectively axially internal and axially external faces, covered by an edge-binding element made of an edge-binding polymer material usually of the same chemical composition as the coating polymer material but which can be a different material. The edge-binding element thus constitutes an additional thickness of polymer coating material at the end of the turned-back portion of carcass reinforcement.
Each bead also comprises a filler element extending the bead wire core radially outwards. The filler element, in any meridian plane, has a substantially triangular cross section and is made of at least one filler polymer material. The filler element may be made of a radial stack of at least two filler polymer materials in contact along a contact surface that intersects any meridian plane along a meridian line. The filler element axially separates the main portion of carcass reinforcement from the turned-back portion of carcass reinforcement.
A polymer material, after curing, is mechanically characterized by tensile stress-deformation characteristics that are determined by tensile testing. This tensile testing is performed by the person skilled in the art on a test specimen, in accordance with a known method, for example in accordance with international standard ISO 37, and under normal temperature (23±2° C.) and moisture (50±5% relative humidity) conditions defined by international standard ISO 471. The tensile stress measured for a 10% elongation of the test specimen is known as the elastic modulus at 10% elongation of a polymer material and is expressed in mega pascals (MPa).
A polymer material, after curing, is also mechanically characterized by its hardness. Hardness is notably defined by the Shore A hardness determined in accordance with ASTM D 2240-86.
As the vehicle drives along, the tire, mounted on its rim, inflated and compressed under the load of the vehicle, is subjected to bending cycles, particularly at its beads and its sidewalls.
The bending cycles lead to variations in curvature combined with variations in tension of the metal reinforcing elements in the main portion of carcass reinforcement and the turned-back portion of carcass reinforcement.
The bending cycles in particular lead to stresses and deformations in the coating, edge-binding and filler polymer materials situated in the immediate vicinity of the end of the turned-back portion of carcass reinforcement and which, over time, are likely to lead to degradation of the tire requiring it to be replaced.
More specifically, the stresses and deformations in the immediate vicinity of the end of the turned-back portion of carcass reinforcement lead to the spread of cracks initiated at the end of the turned-back portion of carcass reinforcement, especially when the reinforcing elements are made of metal.
According to the inventors, the initiation of cracks results chiefly from a lack of adhesion between the ends of the metal reinforcing elements of the turned-back portion of carcass reinforcement and the coating, edge-binding or filler polymer materials in contact with the said ends. The increase in bead temperature, during the bending cycles, accentuates the lack of adhesion that already exists in the new tire.
The cracks spread through the coating, edge-binding and filler polymer materials and lead to degradation of the bead, and therefore failure of the tire. The rate at which the cracks spread is dependent firstly on the amplitude and frequency of the stress and strain deformation cycles and secondly on the respective rigidities of the coating, edge-binding and filler polymer materials in the crack zone.
Document U.S. Pat. No. 3,921,693 has already described, in the case of a tire with a radial carcass reinforcement the reinforcing elements of which are made of metal, beads which have a design aimed at preventing cracks at the end of the turned-back portion of carcass reinforcement. In the technical solution proposed, the end of the turned-back portion of carcass reinforcement is covered with a polymer material the Shore A hardness of which is higher than that of the filler polymer material or materials.
Document U.S. Pat. No. 4,086,948 has also described with a view to increasing the life of a radial tire for a heavy vehicle, a tall turned-back portion of carcass reinforcement, which means the end of which is radially on the outside of the straight line passing through the axially outermost points of the sidewalls of the tire. In addition, the polymer material with the metal reinforcing elements of the carcass reinforcement are coated has a Shore A hardness and an elastic modulus at 300% elongation which are respectively higher than the Shore A hardness and than the elastic modulus at 300% elongation of the filler polymer material.
Finally, document U.S. Pat. No. 5,056,575 describes a tire bead for a heavy vehicle, such as trucks and buses, which allows a reduction in deformations and slows the spread of cracks through the polymer material near the end of the turned-back portion of carcass reinforcement so as to increase the durability of the bead. The technical solution proposed consists of a bead that has three filler polymer materials of which the elastic moduli at 100% elongation decrease from the filler polymer material adjacent to the turned-back portion of carcass reinforcement which is the radially outermost one, to the filler polymer material adjacent to the bead wire core which is the radially innermost one.